Ethylene-selective electrode with a mixed valence cu4o3 catalyst

ABSTRACT

An electrode including Cu4O3, in particular an ethylene-selective electrode with a mixed valence Cu4O3 catalyst. A method for producing an electrode of this type, an electrolytic cell having an electrode of this type, and a method for electrochemically converting carbon dioxide using such an electrode including Cu4O3.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International Application No. PCT/EP2018/078704 filed 19 Oct. 2018, and claims the benefit thereof. The International Application claims the benefit of German Application No. DE 10 2017 220 450.8 filed 16 Nov. 2017. All of the applications are incorporated by reference herein in their entirety.

FIELD OF INVENTION

The present invention relates to an electrode comprising Cu₄O₃, to a process for producing such an electrode, to an electrolysis cell comprising such an electrode, and to a process for electrochemical conversion of carbon dioxide and/or carbon monoxide using such an electrode.

BACKGROUND OF INVENTION

At present, about 80% of global energy demand is covered by the combustion of fossil fuels, the combustion processes of which cause global emission of about 34 000 million tonnes of carbon dioxide into the atmosphere per annum. This release into the atmosphere disposes of the majority of carbon dioxide, which can be up to 50 000 tonnes per day in the case of a brown coal power plant, for example. Carbon dioxide is one of the gases known as greenhouse gases, the adverse effects of which on the atmosphere and the climate are a matter of discussion. It is a technical challenge to produce products of value from CO₂. Since carbon dioxide is at a very low thermodynamic level, it can be reduced to reutilizable products only with difficulty, which has left the actual reutilization of carbon dioxide in the realm of theory or in the academic field to date.

Natural carbon dioxide degradation proceeds, for example, via photosynthesis. This involves conversion of carbon dioxide to carbohydrates in a process subdivided into many component steps over time and, at the molecular level, in terms of space. As such, this process cannot easily be adapted to the industrial scale. No copy of the natural photosynthesis process with photocatalysis on the industrial scale to date has had adequate efficiency.

An alternative is the electrochemical reduction of carbon dioxide. Systematic studies of the electrochemical reduction of carbon dioxide are still a relatively new field of development. Only in the last few years have there been efforts to develop an electrochemical system that can reduce an acceptable amount of carbon dioxide. Research on the laboratory scale has shown that electrolysis of carbon dioxide is advantageously accomplished using metals as catalysts. For example, the publication “Electrochemical CO₂ reduction on metal electrodes by Y. Hori”, published in: C. Vayenas, et al. (eds.), Modern Aspects of Electrochemistry, Springer, New York, 2008, p. 89-189, gives Faraday efficiencies at different metal cathodes which are listed in table 1 below, taken from this publication.

TABLE 1 Faraday efficiencies in the electrolysis of CO₂ over various electrode materials Elec- trode CH₄ C₂H₄ C₂H₅OH C₃H₇OH CO HCOO⁻ H₂ Total Cu 33.3 25.5 5.7 3.0 1.3 9.4 20.5 103.5 Au 0.0 0.0 0.0 0.0 87.1 0.7 10.2 98.0 Ag 0.0 0.0 0.0 0.0 81.5 0.8 12.4 94.6 Zn 0.0 0.0 0.0 0.0 79.4 6.1 9.9 95.4 Pd 2.9 0.0 0.0 0.0 28.3 2.8 26.2 60.2 Ga 0.0 0.0 0.0 0.0 23.2 0.0 79.0 102.0 Pb 0.0 0.0 0.0 0.0 0.0 97.4 5.0 102.4 Hg 0.0 0.0 0.0 0.0 0.0 99.5 0.0 99.5 In 0.0 0.0 0.0 0.0 2.1 94.9 3.3 100.3 Sn 0.0 0.0 0.0 0.0 7.1 88.4 4.6 100.1 Cd 1.3 0.0 0.0 0.0 13.9 78.4 9.4 103.0 Tl 0.0 0.0 0.0 0.0 0.0 95.1 6.2 101.3 Ni 1.8 0.1 0.0 0.0 0.0 1.4 88.9 92.4 Fe 0.0 0.0 0.0 0.0 0.0 0.0 94.8 94.8 Pt 0.0 0.0 0.0 0.0 0.0 0.1 95.7 95.8 Ti 0.0 0.0 0.0 0.0 0.0 0.0 99.7 99.7

The table reports Faraday efficiencies [%] of products that form in the reduction of carbon dioxide at various metal electrodes. The values reported are applicable here to a 0.1 M potassium hydrogencarbonate solution as electrolyte and current densities below 10 mA/cm².

While carbon dioxide is reduced almost exclusively to carbon monoxide at silver, gold, zinc, palladium and gallium cathodes, for example, a multitude of hydrocarbons form as reaction products at a copper cathode.

For example, in an aqueous system, predominantly carbon monoxide and a little hydrogen would form at a silver cathode. The reactions at anode and cathode in that case can be represented by way of example by the following reaction equations:

Cathode: 2 CO₂+4 e ⁻+4 H⁺→2 CO+2 H₂O

Anode: 2 H₂O→O₂+4 H⁺+4 e ⁻

Of particular economic interest, for example, is the electrochemical production of carbon monoxide, ethylene or alcohols.

Examples:

Carbon monoxide: CO₂+2e ⁻+H₂O→CO+2OH⁻

Ethylene: 2 CO₂+12 e ⁻+8 H₂O→C₂H₄+12 OH⁻

Methane: CO₂+8 e ⁻+6 H₂O→CH₄+8 OH⁻

Ethanol: 2 CO₂+12 e ⁻+9 H₂O→C₂H₅OH+12 OH⁻

Monoethylene glycol: 2 CO₂+10 e ⁻+8 H₂O→HOC₂H₄OH+10 OH⁻

The reaction equations show that, for the production of ethylene from CO₂, for example, 12 electrons have to be transferred.

The stepwise reaction of CO₂ proceeds via a multitude of surface intermediates (—CO₂ ⁻, —CO, ═CH₂, —H, etc). For each of these intermediates, there should advantageously be a strong interaction with the catalyst surface or the active sites, such that a surface reaction (or further reaction) between the corresponding adsorbates is enabled. Product selectivity is thus directly dependent on the crystal surface or interaction thereof with the surface species. For example, an elevated ethylene selectivity has been shown by experiments on monocrystalline high-index surfaces (Cu 711, 511) in Journal of Molecular Catalysis A Chemical 199(1):39-47, 2003. Materials that have a high number of crystallographic levels or have surface defects likewise have elevated ethylene selectivities, as shown in C. Reller, R. Krause, E. Volkova, B. Schmid, S. Neubauer, A. Rucki, M. Schuster, G. Schmid, Adv. Energy Mater. 2017, 1602114 (DOI: 10.1002/aenm.201602114), and DE102015203245 A1.

There is thus a close relationship between the nanostructure of the catalyst material and the ethylene selectivity. As well as the property of selectively forming ethylene, the material should retain its product selectivity even at high conversion rates (current densities), i.e. the advantageous structure of the catalyst centers should be conserved. However, owing to high surface mobility of copper, the defects or nanostructures generated typically do not have prolonged stability, and so, even after a short time (60 min), degradation of the electrocatalyst can be observed. As a result of the structural alteration, the material loses the propensity to form ethylene. Moreover, with voltage applied to structured surfaces, the potentials vary easily, such that certain intermediates are formed preferentially in a small area at certain points, and these can then react further at a slightly different point. As in-house studies have shown, potential variations well below 50 mV are significant.

On the basis of numerous studies, it is now generally acknowledged that Cu₂O increases the selectivity of copper for C₂H₄ and hydrocarbons. On the other hand, copper(II) oxide (CuO), owing to its poor properties as catalyst for CO₂ electroreduction, has attracted little attention. Owing to its morphology (nanowires, dendrites, needles, thin layers, particles, etc.) and exposed crystal planes, Cu₂O can be selective for various liquid and gaseous C1 and C2 products. However, stability is still one of the greatest disadvantages for the use of Cu₂O phases in prolonged CO₂ electroreduction, since it is not stable to reduction under operating conditions, as apparent from the Pourbaix diagram for copper.

There are no known catalyst systems to date that have prolonged stability and are capable of electrochemically reducing CO₂ to ethylene at high current density >100 mA/cm². Current densities of industrial relevance can be achieved using gas diffusion electrodes (GDEs). This is known from the prior art, for example, for chlor-alkali electrolyses implemented on the industrial scale.

Cu-based gas diffusion electrodes for production of hydrocarbons on the basis of CO₂ are already known from the literature. In the studies by R. Cook in J. Electrochem. Soc., Vol. 137, No. 2, 1990, for example, a wet-chemical process based on a PTFE 30B (suspension)/Cu(OAc)₂/Vulkan XC 72 mixture is mentioned. The method states how a hydrophobic conductive gas transport layer is applied using three coating cycles, and a catalyst-containing layer using three further coating operations. Each layer is followed by a drying phase (325° C.) with a subsequent static pressing operation (1000-5000 psi). For the electrode obtained, a Faraday efficiency of >60% and a current density of >400 mA/cm² were reported. Reproduction experiments demonstrate that the static pressing method described does not lead to stable electrodes. A disadvantageous effect of the added Vulkan XC 72 was likewise found, and so it was likewise the case that no hydrocarbons were obtained.

The most efficient catalysts for reduction of CO₂ to higher hydrocarbons and ethylene have to date been copper (with various morphologies) and copper(I) oxide (with various morphologies). One example of a current Cu catalyst for CO₂ reduction can be found, for example, in Ma, S. et al., One-step electrosynthesis of ethylene and ethanol from CO₂ in an alkaline electrolyzer, J. Power Sources 301, 219-228 (2016).

However, there is still a need for highly efficient electrodes and electrolysis systems having prolonged stability for efficient preparation of ethylene from carbon dioxide.

SUMMARY OF INVENTION

The inventors have found that Cu₄O₃ is of excellent suitability as a catalyst having prolonged stability for the reduction of carbon dioxide to ethylene. Cu₄O₃ has to date never been used or considered as catalyst for the electrochemical reduction of CO₂. In this respect, what is disclosed in accordance with the invention is that Cu₄O₃ is used as catalyst for the electrochemical reduction of CO₂. In addition, Cu₄O₃ may also be just a catalyst constituent. The Cu₄O₃ can also be used as pre-catalyst. Furthermore, under acidic conditions, reduction with dendrite formation is possible. More particularly, a gas diffusion electrode comprising Cu₄O₃ is disclosed as electrocatalyst for CO₂ reduction that exhibits high activity (>400 mA/cm²) and selectivity for ethylene.

The inventors have found more particularly that advantageously gas diffusion electrodes or layers, advantageously with at least 0.5 mg/cm² of Cu₄O₃ catalyst, have the following advantages in the electrochemical reduction of CO₂ to hydrocarbons:—higher selectivity for ethylene compared to Cu, Cu₂O and CuO;—higher stability at reduction potential against reduction to Cu;—superior activity compared to Cu, Cu₂O and CuO; and—a lower overvoltage for the reduction of CO₂ to ethylene compared to Cu, Cu₂O and CuO.

In a first aspect, the present invention relates to an electrode, especially for an electrochemical conversion in liquid electrolytes, comprising Cu₄O₃.

Also disclosed is an electrolysis cell comprising the electrode of the invention, advantageously as cathode.

Likewise disclosed is a process for producing an electrode comprising Cu₄O₃ on a support, comprising—preparing a mixture comprising Cu₄O₃ and optionally at least one binder, or providing a powder consisting of Cu₄O₃,—applying the mixture comprising Cu₄O₃ or the powder consisting of Cu₄O₃ to a, for example copper-containing, support, advantageously in the form of a sheetlike structure, and—dry rolling the mixture comprising Cu₄O₃ or the powder consisting of Cu₄O₃ onto the support.

Also disclosed is a process for producing an electrode comprising Cu₄O₃ on a substrate, comprising—providing a support,—applying a suspension comprising Cu₄O₃ and optionally at least one binder to the support, and—drying the suspension.

The present invention additionally also relates to a process for producing an electrode comprising Cu₄O₃, comprising—preparing a powder comprising Cu₄O₃; and—rolling the powder out to give an electrode.

These processes of the invention can especially be used to produce an electrode of the invention.

The present invention still further comprises a process for electrochemical conversion of carbon dioxide and/or carbon monoxide, wherein carbon dioxide and/or carbon monoxide is introduced at the cathode into an electrolysis cell comprising an electrode of the invention as cathode and reduced.

The present invention additionally relates to the use of Cu₄O₃ for reduction of CO₂, and to the use of Cu₄O₃ in the electrolysis of CO₂.

Further aspect of the present invention can be taken from the dependent claims and the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The appended drawings are intended to illustrate embodiments of the present invention and impart further understanding thereof. In association with the description, they serve to elucidate concepts and principles of the invention. Other embodiments and many of the advantages mentioned are apparent with regard to the drawings. The elements of the drawings are not necessarily shown true to scale relative to one another. Elements, features and components that are the same, have the same function and the same effect are each given the same reference signs in the figures of the drawings, unless stated otherwise.

The electrochemical stability of paramelaconite, Cu₄O₃, is shown in a Pourbaix diagram in FIG. 1.

FIGS. 2 to 19 (FIGS. 17-19 with feed and removal devices, etc.) show electrolysis cells in schematic form, in which the electrode of the invention, especially in the form of a gas diffusion electrode or a gas diffusion layer, can be employed, and which are thus possible embodiments of an electrolysis cell of the invention.

FIGS. 20 and 21 show measurement results of data recorded with a powder x-ray diffractometer (PXRD) of powder obtained in the examples comprising Cu₄O₃, and FIGS. 22 and 23 show images of the powder with a scanning electron microscope (SEM).

FIGS. 24 to 31 show measurement results that have been obtained in inventive examples and comparative examples in electrolysis cells in the reduction of CO₂.

DETAILED DESCRIPTION OF THE INVENTION Definitions

Unless defined differently, technical and scientific terms used herein have the same meaning as commonly understood by a person skilled in the art in the specialist field of the invention.

An electrode is an electrical conductor that can supply electrical current to a liquid, gas, vacuum or a solid state body. More particularly, an electrode is not a powder or a particle, but may comprise particles and/or a powder or be produced from a powder. A cathode here is an electrode at which an electrochemical reduction can take place, and an anode an electrode at which an electrochemical oxidation can take place. The electrochemical conversion takes place here, in particular embodiments, in the presence of advantageously aqueous electrolytes.

Stated amounts in the context of the present invention relate to % by weight, unless stated otherwise or apparent from the context. In the gas diffusion electrode of the invention, the percentages by weight add up to 100% by weight.

“Hydrophobic” in the context of the present invention is understood to mean water-repellent. According to the invention, hydrophobic pores and/or channels are thus those that repel water. More particularly, hydrophobic properties are associated according to the invention with substances or molecules having nonpolar groups.

“Hydrophilic”, by contrast, is understood to mean the ability to interact with water and other polar substances.

The present invention relates, in a first aspect, to an electrode comprising Cu₄O₃. In particular embodiments, the electrode of the invention is a cathode, i.e. can be connected as cathode. In particular embodiments, Cu₄O₃ is used as catalyst for the electrochemical reduction of CO₂. In addition, the Cu₄O₃, in particular embodiments, may also be a catalyst constituent. The Cu₄O₃, in particular embodiments, may also be used as pre-catalyst. Under acidic conditions, in addition, reduction with dendrite formation is possible, such that the electrode of the invention, in particular embodiments, may comprise the Cu₄O₃ in the form of dendrites, although these are not particularly restricted.

Paramelaconite (Cu₄O₃), together with copper(I) oxide (Cu₂O) and copper(II) oxide (CuO), forms part of the copper oxide family. Although Cu₂O and CuO have been studied in detail, less is known about Cu₄O₃ since it is rare and its synthesis is complex. Cu₃O₄ is a metastable phase which is not directly obtainable by thermal oxidation of oxygen-free copper.

The copper-oxygen system is an example of a simple eutectic system. Oxygen-rich copper contains from 0.01% to 0.05% by weight of oxygen, but may contain up to 0.1% by weight. The solidification thereof commences with core formation on cooling below the liquidus temperature. With falling temperature, these cores that are essentially pure copper become ever larger and the liquid becomes richer in oxygen. The remaining Cu-oxygen environment in the solid phase can form a tetragonal structure comparable to the Cu₄O₄ structure, or form Cu. In copper refining, for example, air is injected into the melt in order to oxidize impurities, and oxygen can be absorbed by the copper in this step. In the refining of copper, oxygen and hydrogen counteract one another. The oxygen content can often be removed by chamfering. Oxygen-rich copper showed much higher Faraday efficiencies for ethylene in the electroreduction of CO₂. This effect could be connected to remaining oxygen species beneath the surface, as described by A. Eilert, J. Phys. Chem. 2017, 8 (1), pp. 285-290. Nevertheless, Cu₄O₃ and similar coordination compounds between copper and oxygen have not been taken into account to date in this regard.

A phase diagram for copper-oxygen <55 at % can be found, for example, in Landoll-Börnstein—Group IV Physical Chemistry Volume 5D: in Springer Materials A Predel, B. E Madelung, Springer-Verlag Berlin Heidelberg 1994, p. 1097, and an oxygen pressure-temperature phase diagram, for example, in Landolt-Börnstein—Group IV Physical Chemistry Volume 5D: in Springer Materials A Predel, B. E Madelung, Springer-Verlag Berlin Heidelberg 1994, p. 1097.

Cu₄O₃ is a mixed-valency oxide having equal proportions of mono- and divalent Cu ions and is therefore sometimes also formally written as Cu⁺2Cu²⁺ ₂O₃. The crystal structure (l4₁/amd space group) of paramelaconite has been identified as tetragonal, consisting of interpenetrating chains of Cu⁺—O and Cu²⁺—O. The Cu²⁺ ions are coordinated to two O²⁻ ions, while the Cu⁺ ions have planar coordination to four O²⁻ ions. Paramelaconite is thermodynamically stable below 300° C.; at temperatures above 300° C. it breaks down to CuO and Cu₂O.

The electrochemical stability of paramelaconite is shown in the Pourbaix diagram in FIG. 1. The diagram shows the higher electrochemical stability of Cu₄O₃ to reduction by comparison with Cu₂O. As apparent from the diagram, a advantageous operating range of electrodes comprising paramelaconite is between pH=6-14 or better between 10 and 14.

Although paramelaconite was first discovered in the 1870s, the controlled synthesis of Cu₄O₃ crystals with reliable phase purity using conventional aqueous chemistry has been a challenge to date, since it is difficult to stabilize Cu²⁺ and Cu⁺ simultaneously. There have been multiple attempts to date to synthesize paramelaconite from the liquid phase, but all these methods suffered from a low paramelaconite yield and small crystal size. It is obtainable by a solvothermal method by prolonged thermal oxidation of copper under air in the presence of boiling aqueous NH₃, as described in P. Morgan, Journal of Solid State Chemistry, 121, 1, 5 Jan. 1996, pages 33-37.

Furthermore, in such processes, only microscopically small amounts of Cu₄O₃ are prepared, which are simultaneously also highly contaminated with CuO and Cu₂O. In 2012, Zhao et al. in Zhao, L. et al., Facile Solvothermal Synthesis of Phase-Pure Cu₄O₃ Microspheres and Their Lithium Storage Properties, Chem. Mater. 2012, 24, pages 1136-1142, described the synthesis of single-phase paramelaconite microspheres by a simple solvothermal method. The Cu₄O₃ microspheres were obtained by reacting the copper(II) nitrate trihydrate precursor (Cu(NO₃)₂.3H₂O) in a mixed solvent composed of ethanol and N,N-dimethylformamide (DMF). The reaction was conducted in a 50 mL Teflon-lined stainless steel autoclave at 130° C. over several hours. As shown in the examples, the inventors were able, by a synthesis along the route followed by Zhao et al., to increase the reaction volume to 1.1 L and to increase the yield to more than 10 g.

In the electrode of the invention, the amount of Cu₄O₃ is not particularly restricted. In particular embodiments, the Cu₄O₃ is present in an amount of 0.1-100% by weight, advantageously 40-100% by weight, more advantageously 70-100% by weight based on the electrode. In particular embodiments, the Cu₄O₃ is present in an amount of 0.1-100% by weight, advantageously 40-100% by weight, more advantageously 70-100% by weight, based on the catalytically active part of the electrode, for example in a layer of the electrode of the invention, for example when the electrode of the invention is in multilayer form, for example with a gas diffusion layer, and/or in the form of a gas diffusion electrode.

In particular embodiments, Cu₄O₃ has been applied to a support which is not particularly restricted, either with regard to the material or to the composition. A support here may, for example, be a compact solid-state body, for example in the form of a pin or strip, for example a metal strip, for example comprising a metal such as Cu or an alloy thereof or consisting of a metal such as Cu or an alloy thereof, or a porous structure, for example a sheetlike structure such as a mesh, a knit, etc., or a coated body. The support may also take the form, for example, of a gas diffusion electrode, optionally also having multiple, e.g. 2, 3, 4, 5, 6 or more, layers of a suitable material or of a gas diffusion layer on a suitable substrate, which is likewise not particularly restricted and may likewise comprise multiple layers, e.g. 2, 3, 4, 5, 6 or more. The gas diffusion electrode or gas diffusion layer used may correspondingly also be a commercially available electrode or layer. The support material is advantageously conductive and comprises, for example, a metal and/or an alloy thereof, a ceramic, for example ITO, an inorganic nonmetallic conductor such as carbon and/or a conductive polymer.

It is of course not impossible that the Cu₄O₃ is also used in the production of gas diffusion layers or gas diffusion electrodes. Thus, an electrode of the invention in particular embodiments is a gas diffusion electrode or an electrode comprising a gas diffusion layer, wherein the gas diffusion electrode or gas diffusion layer contains or even consists of Cu₄O₃. If a gas diffusion layer comprising Cu₄O₃ is present, this may have been applied to a porous or nonporous substrate.

When the Cu₄O₃ has been applied to a support, in particular embodiments, it has been applied with a mass coverage of at least 0.5 mg/cm². The application here is advantageously not two-dimensional, in order to be able to provide a greater active surface area. Moreover, the application advantageously forms pores, or pores of the support are essentially not closed, such that a gas such as carbon dioxide can easily reach the Cu₄O₃. In particular embodiments, the Cu₄O₃ has been applied with a mass coverage between 0.5 and 20 mg/cm², advantageously between 0.8 and 15 mg/cm², more advantageously between 1 and 10 mg/cm². Proceeding from these values, the amount of Cu₄O₃ as catalyst can be suitably determined for application to a particular support.

The inventors have more particularly found that gas diffusion electrodes or layers in particular, advantageously with at least 1 mg/cm² of Cu₄O₃ catalyst, have the following advantages in the electrochemical reduction of CO₂ to hydrocarbons:—higher selectivity for ethylene compared to Cu, Cu₂O and CuO;—higher stability at reaction potential against reduction to Cu;—superior activity compared to Cu, Cu₂O and CuO; and—a lower overvoltage for the reduction of CO₂ to ethylene compared to Cu, Cu₂O and CuO.

In particular embodiments, the electrode of the invention is a gas diffusion electrode which is not particularly restricted and may be in single- or multilayer form, for example with 2, 3, 4, 5, 6 or more layers. In such a multilayer gas diffusion electrode, it is then possible, for example, for the Cu₄O₃ also to be present solely in one layer or not in all layers, i.e., for example, to form one or more gas diffusion layers. Especially with a gas diffusion electrode, good contacting with a gas comprising CO₂ or consisting essentially of CO₂ is very efficiently possible, such that efficient electrochemical preparation of C₂H₄ can be achieved here. Furthermore, this can alternatively be brought about with an electrode comprising a gas diffusion layer comprising or consisting of Cu₄O₃, since a large reaction area can also be offered here to such a gas.

In particular, the following important specific parameters and properties of a hydrocarbon-selective gas diffusion electrode or gas diffusion layer have been found:

-   -   Good wettability of the electrode surface in order that an         aqueous electrolyte or H+ ions can come into contact with         catalyst (H+ is required for ethylene).     -   High electrical conductivity of the electrode or of the catalyst         and a homogeneous potential distribution over the entire         electrode area (potential-dependent product selectivity).     -   High chemical and mechanical stability in electrolysis operation         (suppression of cracking and corrosion).     -   The ratio between hydrophilic and hydrophobic pore volume is         advantageously in the region of about (0.01-1):3, more         advantageously approximately in the region of (0.1-0.5):3 and         advantageously about 0.2:3.     -   Defined porosity with a suitable ratio between hydrophilic and         hydrophobic channels or pores (assurance of CO₂ availability in         the simultaneous presence of H+ ions).

Average pore sizes in the range from 0.2 to 7 μm, advantageously in the range from 0.4 to 5 μm and more advantageously in the range from 0.5 to 2 μm have also been found to be advantageous in a gas diffusion electrode or a gas diffusion layer.

In particular embodiments, catalyst particles comprising or consisting of Cu₄O₃, for example Cu₄O₃ particles, that are used for production of the electrode of the invention, especially a gas diffusion electrode, or a gas diffusion layer have a uniform particle size, for example between 0.01 and 100 μm, for example between 0.05 and 80 μm, advantageously 0.08 to 10 μm, more advantageously between 0.1 and 5 μm, for example between 0.5 and 1 μm. Moreover, the catalyst particles, in particular embodiments, also have a suitable electrical conductivity, especially a high electrical conductivity σ of >10³ S/m, advantageously 10⁴ S/m or more, more advantageously of 10⁵ S/m or more, especially 10⁶ S/m or more, where suitable additives may optionally be added here to the paramelaconite in order to increase the conductivity of the particles, for example metal particles. In addition, the catalyst particles, in particular embodiments, have a low overvoltage for the electroreduction of CO₂. In addition, the catalyst particles, in particular embodiments, have a high purity without traces of extraneous metal. By suitable structuring, optionally with the aid of promoters and/or additives, it is possible to achieve high selectivity and prolonged stability.

For a particularly good catalytic activity, a gas diffusion electrode or an electrode with a gas diffusion layer should have hydrophilic and hydrophobic regions that enable a good relationship between the three phases: liquid, solid, gaseous. Particularly active catalyst sites are in the three-phase region of liquid, solid, gaseous. An ideal gas diffusion electrode thus has penetration of the bulk material with hydrophilic and hydrophobic channels in order to obtain a maximum number of three-phase regions for active catalyst sites. Similarly, a gas diffusion layer should correspondingly also have hydrophilic and hydrophobic channels.

For hydrocarbon-selective gas diffusion electrodes and gas diffusion layers, accordingly, multiple intrinsic properties are needed. There is a close interplay between the electrocatalyst and the electrode.

It is not ruled out in accordance with the invention that the electrode of the invention, as well as Cu₄O₃, also comprises further constituents such as promoters, conductivity additives, co-catalysts and/or binding agents/binders (the terms binding agent and binder are treated as synonymous words with the same meaning in the context of the present invention). For example, as specified above, it is possible to add additives to increase the conductivity, in order to enable good electrical and/or ionic contacting of the Cu₄O₃. Co-catalysts may, for example, optionally catalyze the formation of further products from ethylene and/or else the formation of intermediates in the electrochemical reduction of CO₂ to ethylene, but may also possibly catalyze entirely different reactions, for example when a reactant other than CO₂ is used in an electrochemical reaction, for example an electrolysis.

The electrode of the invention in particular, especially a gas diffusion electrode or a gas diffusion layer, may include at least one binder, which is not particularly restricted, and it is also possible to use two or more different binders, including in different layers of the electrode. The binding agent or binder for the gas diffusion electrode of the invention, if present, is not particularly restricted and includes, for example, a hydrophilic and/or hydrophobic polymer, for example a hydrophobic polymer. This can achieve a suitable adjustment of the predominantly hydrophobic pores or channels. In particular embodiments, the at least one binder is an organic binder, for example selected from PTFE (polytetrafluoroethylene), PVDF (polyvinylidene difluoride), PFA (perfluoroalkoxy polymers), FEP (fluorinated ethylene-propylene copolymers), PFSA (perfluorosulfonic acid polymers), and mixtures thereof, especially PTFE. The hydrophilicity can also be adjusted using hydrophilic materials such as polysulfones, i.e. polyphenylsulfones, polyimides, polybenzoxazoles or polyetherketones, or generally polymers that are electrochemically stable in the electrolyte, for example including polymerized “ionic liquids”, or organic conductors such as PEDOT:PSS or PANI (camphorsulfonic acid-doped polyaniline). This can achieve a suitable adjustment of the hydrophobic pores or channels. More particularly, the gas diffusion electrode can be produced using PTFE particles having a particle diameter between 0.01 and 95 μm, advantageously between 0.05 and 70 μm, more advantageously between 0.1 and 40 μm, e.g. 0.3 to 20 μm, e.g. 0.5 to 20 μm, e.g. about 0.5 μm. Suitable PTFE powders include, for example, Dyneon® TF 9205 and Dyneon TF 1750. Suitable binder particles, for example PTFE particles, may, for example, be approximately spherical, for example spherical, and may be produced, for example, by emulsion polymerization. In particular embodiments, the binder particles are free of surface-active substances. The particle size can be determined here, for example, to ISO 13321 or D4894-98a and may correspond, for example, to manufacturer data (e.g. TF 9205: average particle size 8 μm to ISO 13321; TF 1750: average particle size 25 μm to ASTM D4894-98a).

The binder may be present, for example, in a proportion of 0.1% to 50% by weight, for example when a hydrophilic ion transport material is used, e.g. 0.1% to 30% by weight, advantageously from 0.1% to 25% by weight, e.g. 0.1% to 20% by weight, more advantageously from 3% to 20% by weight, more advantageously 3% to 10% by weight, even more advantageously 5% to 10% by weight, based on the gas diffusion electrode. In particular embodiments, the binder has significant shear-thinning characteristics, such that fiber formation takes place during the mixing process. Ion transport materials may be mixed in, for example, with higher contents when they contain hydrophobic or hydrophobizing structural units especially containing F, or fluorinated alkyl or aryl units. Fibers formed in the course of production should ideally wind around the particles without completely surrounding the surface. The optimal mixing time can be determined, for example, by direct visualization of the fiber formation in a scanning electron microscope.

It is also possible to employ an ion transport material in the electrode of the invention, which is not particularly restricted. The ion transport material, for example an ion exchange material, is not particularly restricted in accordance with the invention and may, for example, be an ion transport resin, for example an ion exchange resin, or else a different ion transport material, for example an ion exchange material, for example a zeolite, etc. In particular embodiments, the ion transport material is an ion exchange resin. This is not particularly restricted here. In particular embodiments, the ion transport material is an anion transport material, for example an anion exchange resin. In particular embodiments, the anion transport material or anion transporter is an anion exchange material, for example an anion exchange resin. In particular embodiments, the ion transport material also has a cation blocker function, i.e. can prevent or at least reduce penetration of cations into the electrode, especially a gas diffusion electrode or an electrode having a gas diffusion layer. Specifically an integrated anion transporter or an anion transport material with firmly bound cations can constitute a barrier here to mobile cations through coulombic repulsion, which can additionally counteract salt deposition, especially within a gas diffusion electrode or a gas diffusion layer. It is unimportant here whether the gas diffusion electrode is fully permeated by the anion transporter. Anion-conducting additives which are not particularly restricted can additionally increase the performance of the electrode, especially in a reduction. For example, it is possible here to use an ionomer, for example 20% by weight alcoholic suspension or a 5% by weight suspension of an anion exchanger monomer (e.g. AS 4 Tokuyama). It is also possible, for example, to use type 1 (typically trialkylammonium-functionalized resins) and type 2 (typically alkylhydroxyalkyl-functionalized resins) anion exchange resins.

In a further aspect, the present invention relates to an electrolysis cell comprising the electrode of the invention. The electrode may take the form here of a compact solid-state body, of a porous electrode, e.g. gas diffusion electrode, or of a coated body, for example with a gas diffusion layer, preference being given to executions as a gas diffusion electrode or electrode having a gas diffusion layer comprising or consisting of Cu₄O₃. In the electrolysis cell of the invention, the electrode of the invention is advantageously the cathode, in order to enable reduction, for example, of a gas comprising or consisting of CO₂ and/or CO.

The further constituents of the electrolysis cell are not particularly restricted, and include those that are commonly used in electrolysis cells, for example a counterelectrode.

In particular embodiments, the electrode of the invention in the electrolysis cell is a cathode, i.e. connected as cathode. In particular embodiments, the electrolysis cell of the invention further comprises an anode and at least one membrane and/or at least one diaphragm between the cathode and anode, for example at least one anion exchange membrane.

The further constituents of the electrolysis cell, for instance the counterelectrode, e.g. the anode, optionally a membrane and/or a diaphragm, feed(s) and drain(s), the voltage source, etc., and further optional apparatuses such as heating or cooling devices, are not particularly restricted in accordance with the invention, nor are anolytes and/or catholytes that are used in such an electrolysis cell, with use of the electrolysis cell in particular embodiments on the cathode side for reduction of carbon dioxide and/or CO. In the context of the invention, the configuration of the anode space and of the cathode space is likewise not particularly restricted.

An electrolysis cell of the invention may likewise be employed in an electrolysis system. An electrolysis system is thus also specified, comprising the electrode of the invention or the electrolysis cell of the invention.

A suitable electrolysis cell for the use of the electrode of the invention, for example gas diffusion electrode, comprises, for example, the electrode of the invention as cathode with an anode that is not subject to any further restriction. The electrochemical conversion at the anode/counterelectrode is likewise not particularly restricted. The cell is advantageously divided by the electrode according to the invention as gas diffusion electrode or as electrode having a gas diffusion layer into at least two chambers, of which the chamber remote from the counterelectrode (behind the GDE) functions as gas chamber. One or more electrolytes may flow through the remainder of the cell. The cell may also comprise one or more separators, such that the cell may also comprise, for example, 3 or 4 chambers. These separators may be either gas separators (diaphragms) having no intrinsic ion conductivity or else ion-selective membranes (anion exchange membrane, cation exchange membrane, proton exchange membrane) or bipolar membranes, which are not particularly restricted. It is possible for one or more electrolytes to flow across these separators from both sides, or else, if they are suitable for this kind of operation, for the separators to directly adjoin one of the electrodes. For example, both the cathode and the anode may be executed as a half-membrane electrode composite, where, in the case of the cathode, the electrode of the invention, especially as a gas diffusion electrode or as an electrode with a gas diffusion layer, is advantageously part of this composite. The counterelectrode may also be executed, for example, as a catalyst-coated membrane. In a two-chamber cell, it is also possible for both electrodes to directly adjoin a common membrane. If the electrode of the invention as a gas diffusion electrode does not directly adjoin a separator membrane, either “flow-through” operation in which the feed gas flows through the electrode or “flow-by” operation in which the feed gas is guided past the side remote from the electrolyte is possible. If the gas diffusion electrode directly adjoins the separator or one of the separators, accordingly, only “flow-by” operation is possible. Reference is made to “flow-by” particularly when more than 95% by volume, advantageously more than 98% by volume, of the product gases is discharged via the gas side of the electrode.

Illustrative configurations for a construction of general electrolysis cells—including in accordance with the above remarks—and of possible anode and cathode spaces are shown in schematic form in FIGS. 2 to 19, with further constituents for the purposes of an electrolysis system shown in schematic form in FIGS. 17 to 19. The part that follows especially shows illustration of electrolysis cell concepts that are compatible with the process of the invention for electrochemical conversion of carbon dioxide and/or carbon monoxide.

The following abbreviations are used in FIGS. 1 to 19:

I-IV: spaces in the electrolysis cell, as respectively described hereinafter

K: cathode

M: membrane

A: anode

AEM: anion exchange membrane

CEM: cation/proton exchange membrane

DF: diaphragm

k: catholyte

a: anolyte

GC: gas chromatograph

GH: gas humidification

P: permeate

The other symbols in the diagrams are standard fluidic connection symbols.

The figures show illustrative constructions with different membranes, but these are not intended to restrict the cells shown. For instance, rather than a membrane, it is also possible to provide a diaphragm. The figures also show, on the cathode side, a reduction of a gas, for example comprising or essentially consisting of CO₂, where the electrolysis cells are also not restricted thereto and, accordingly, reactions on the cathode side in the liquid phase or solution, etc., are also possible. In this regard too, the figures do not restrict the electrolysis cell of the invention. It is likewise possible for anolytes, catholytes and any electrolytes in an interspace in the various constructions to be the same or different, and they are not particularly restricted.

FIG. 2 shows an arrangement in which both the cathode K and the anode A adjoin a membrane M, and a reaction gas flows past the back of the cathode K in the cathode space I. On the anode side is the anode space II. In FIG. 3, by comparison with FIG. 2, there is no membrane, and cathode K and anode A are separated by the space II. The construction in FIG. 4, in terms of its construction, corresponds essentially to that of FIG. 3, except that the cathode K here is in flow-through mode.

FIG. 5 shows a two-membrane arrangement, wherein a bridge space II is provided between two membranes M, which electrolytically couples the cathode K and the anode A. The cathode space I corresponds to that of FIG. 1, and the anode space III to the anode space II of FIG. 1. The arrangement in FIG. 6 differs from that of FIG. 5 in that the anode A does not adjoin the second membrane M on the right.

FIGS. 7 to 11 again show arrangements with just one membrane. In FIG. 7, as in FIG. 1, the cathode K in space I is in flow-by mode, while a cathode space II adjoins the membrane M on the other side. The membrane M is in turn separated from the anode A by the anode space III. The construction in FIG. 8 corresponds to that in FIG. 7, except that the cathode K here is in flow-through mode. In FIGS. 9 and 10, the membrane M directly adjoins the anode A, such that the anode space III is on the side of the anode A remote from the membrane M; otherwise, these respectively show the flow-by and flow-through variant of FIGS. 7 and 8. FIG. 11 shows a flow-by variant in which the membrane M adjoins the cathode, space II establishes electrolytic contact with the anode A, and space III is on the opposite side of the anode A.

FIGS. 12 to 16 show further variants of two-membrane arrangements with flow-by variants of the cathode in FIGS. 12, 14 and 16, and flow-through variants in FIGS. 13 and 15. In FIGS. 12 and 13, a membrane (on the right) adjoins the anode, such that the anode space IV adjoins the anode on the right and coupling to the cathode space II takes place via the bridge space III. Such coupling likewise takes place in FIGS. 14 and 15, where the anode space IV here lies between membrane M and anode A. In FIG. 16, in turn, a membrane M (on the left) adjoins the cathode K, such that coupling to the anode space III via the bridge space II is envisaged, with a further space IV provided to the right of the anode A, in which, for example, a further reactant gas for oxidation at the anode A can be supplied.

FIGS. 17 to 19 show cell variants in which, by way of example, reduction of CO₂ at the cathode K after supply to space I and oxidation of water at the anode A—which is supplied to the anode space III with the anolyte a—to oxygen is shown, where these reactions do not restrict the electrolysis cells and electrolysis systems shown. FIGS. 17 and 18 additionally show that the CO₂ can be humidified in a gas humidification GH, in order to facilitate ionic contacting with the cathode K. In addition, as shown in FIGS. 17 to 19, the product gas from the reduction can additionally be analyzed with a gas chromatograph GC. The same applies, as shown in FIGS. 17 and 18, after removal of a permeate p for the reactant gas. In FIG. 17, a catholyte k is supplied to the bridge space II, which enables electrolytic coupling between cathode K and anode A, with the cathode K adjoining an anion exchange membrane AEM and the anode A adjoining a cation exchange membrane CEM. In FIG. 18, only a cation exchange membrane CEM is present; otherwise, the construction corresponds to that of FIG. 17, except that the space II here is in direct contact with the cathode K, i.e. does not constitute a bridge space. In the cell construction of FIG. 19, by comparison with FIG. 18, the cation exchange membrane CEM does not adjoin the anode.

In addition, there are also possible cell variants as already described in DE 10 2015 209 509 A1, DE 10 2015 212 504 A1, DE 10 2015 201 132 A1, DE 102017208610.6, DE 102017211930.6, US 2017037522 A1 or U.S. Pat. No. 9,481,939 B2, and in which an electrode of the invention may likewise be employed.

As apparent from the above, the present electrode results in a multitude of possible cell arrangements.

The aspects that follow relate to various production processes for producing an electrode. The processes of the invention can especially produce an electrode of the invention, such that elucidations relating to particular constituents of the electrode can also be applied to the processes.

A further aspect of the present invention relates to a process for producing an electrode comprising Cu₄O₃ on a support, comprising—preparing a mixture comprising Cu₄O₃ and optionally at least one binder, or providing a powder consisting of Cu₄O₃,—applying the mixture comprising Cu₄O₃ or the powder consisting of Cu₄O₃ to a support, for example a copper-containing support, advantageously in the form of a sheetlike structure, and—dry rolling the mixture comprising Cu₄O₃ or the powder consisting of Cu₄O₃ onto the support. More particularly, this process, like the other processes of the invention, as indicated above, can be used to produce an electrode of the invention.

For processing of a mixture, for example a powder mixture, or of a powder to give an electrode, especially a gas diffusion electrode or an electrode with a gas diffusion layer, it is possible, for example, to employ the dry calendering process described in DE 102015215309.6 or WO 2017/025285. In this respect, with regard to the production process by dry calendering, reference is also made to this application.

The preparation of the mixture comprising Cu₄O₃ and optionally at least one binder is not particularly restricted here and can be effected in a suitable manner. For example, the mixing can be effected with a knife mill, but is not limited thereto. A advantageous mixing time in a knife mill is in the range of 60-200 s, advantageously between 90-150 s. Other mixing times may correspondingly also arise for other mixers. In particular embodiments, however, the preparing of the mixture comprises mixing for 60-200 s, advantageously 90-150 s.

The applying of the mixture or the powder to a support, for example a copper-containing support, advantageously in the form of a sheetlike structure, is likewise not particularly restricted and can be effected, for example, by applying in powder form, etc. The support here is not particularly restricted and may correspond to the above descriptions with regard to the electrode, and it may be executed here, for example, as a mesh, grid, etc.

The dry rolling of the mixture or the powder onto the support is not particularly restricted either, and can be effected, for example, with a roller. In particular embodiments, the rolling application is effected at a temperature of 25-100° C., advantageously 60-80° C.

Nor is it ruled out in accordance with the invention that multiple layers are collectively applied and rolled onto a support by this process, for example a hydrophobic layer that can establish better contact with a gas comprising CO₂ and hence can improve gas transport to the catalyst.

It is also possible for the catalyst, i.e. Cu₄O₃, to be sieved onto an existing electrode without an additional binder. The base layer may then also be produced, for example, from powder mixtures of a Cu powder, for example with a grain size of 100-160 μm, with a binder, e.g. 10-15% by weight of PTFE Dyneon TF 1750 or 7-10% by weight of Dyneon TF 2021.

A further aspect of the present invention relates to a process for producing an electrode comprising Cu₄O₃ on a support, comprising—providing a support,—applying a suspension comprising Cu₄O₃ and optionally at least one binder to the support, and—drying the suspension; or—providing a support, and—applying Cu₄O₃ or a mixture comprising Cu₄O₃ from the gas phase. More particularly, it is also possible by this process, like the other processes of the invention too, to produce an electrode of the invention.

Here too, the providing of the support is not particularly restricted, and it is possible to use, for example, the support discussed in the context of the electrode, for example including a gas diffusion electrode or gas diffusion layer, for example on a suitable substrate. The applying of the suspension is likewise not particularly restricted, and can be effected, for example, by dropwise application, dipping, etc. The material may thus be applied, for example, as a suspension to a commercially available GDL (e.g. Freudenberg C2, Sigracet 35 BC). It is preferable when an ionomer, for example 20% by weight alcoholic suspension or a 5% by weight suspension of an anion exchange ionomer (e.g. AS 4 Tokuyama), is also used here, and/or other additives, binders, etc., that have been discussed in the context of the electrode of the invention. For example, it is also possible to use type 1 (typically trialkylammonium-functionalized resins) and type 2 (typically alkylhydroxyalkyl-functionalized resins) anion exchange resins.

The drying of the suspension is likewise not restricted and it is possible, for example, to effect solidification by evaporation or precipitation with separation of the solvent or solvent mixture from the suspension, which are not particularly restricted.

In the alternative embodiment of the applying of Cu₄O₃ or a mixture comprising Cu₄O₃ from the gas phase, the providing of a support is likewise not particularly restricted, and can be effected as above. The applying of Cu₄O₃ or of a mixture comprising Cu₄O₃ from the gas phase is likewise not particularly restricted and can be effected, for example, based on physical gas phase deposition methods such as laser ablation or chemical vapor deposition (CVD). In this way, it is possible to obtain thin films comprising paramelaconite.

In particular embodiments, the support is a gas diffusion electrode or a gas diffusion layer.

In the processes specified above in which not only paramelaconite but also other constituents may be present in a mixture or suspension, in particular embodiments, the at least one binder is present in the mixture or suspension, wherein the at least one binder advantageously comprises an ionomer. In particular embodiments, the at least one binder is present in the mixture or suspension in an amount of >0% to 30% by weight, based on the total weight of Cu₄O₃ and the at least one binder.

A further aspect relates to a process for producing an electrode comprising Cu₄O₃, comprising a preparation of a powder comprising Cu₄O₃; and rolling out the powder to give an electrode. The production of the powder comprising Cu₄O₃ is not particularly restricted here, nor is the rolling-out to give a powder, for example with a roller. The rolling-out can be effected, for example, at a temperature of 15 to 300° C., e.g. 20 to 250° C., e.g. 22 to 200° C., advantageously 25-150° C., further advantageously 60-80° C. With regard to the powder, it is again also possible to make reference to the embodiments above relating to the electrode of the invention. This process can especially, like the other processes of the invention as well, be used to produce an electrode of the invention.

A further aspect of the present invention is a process for electrochemical conversion of carbon dioxide and/or carbon monoxide, wherein carbon dioxide and/or carbon monoxide are introduced at the cathode into an electrolysis cell—comprising an electrode of the invention as cathode—and reduced.

The present invention thus also relates to a process and to an electrolysis system for electrochemical utilization of carbon dioxide. Carbon dioxide (CO₂) is introduced into an electrolysis cell and reduced at a cathode with the aid of an electrode of the invention, for example a gas diffusion electrode (GDE), on the cathode side. GDEs are electrodes in which liquid, solid and gaseous phases are present and where the conductive catalyst catalyzes the electrochemical reaction between the liquid phase and the gaseous phase.

The introducing of the carbon dioxide and/or optionally also carbon monoxide at the cathode is not particularly restricted here, and can be effected from the gas phase, from solution, etc.

In order to assure sufficiently high conductivity in the cathode space, an aqueous electrolyte in contact with the cathode, in particular embodiments, contains a dissolved “conductive salt” which not particularly restricted. The electrocatalyst used should ideally enable a high Faraday efficiency at high current density for a corresponding target product. Industrially relevant electrocatalysts should additionally have prolonged stability. For the selective production of the carbon monoxide product, pure silver catalyst that meet industrial demands are already available. For the selective electroreduction of CO₂ to ethylene or alcohols, there are currently no known catalysts available that meet these demands. The synthesis concept described here enables the production of electrocatalysts with a low overvoltage and an elevated selectivity for ethylene and alcohols, for example ethanol and/or propanol.

In particular embodiments, the electrochemical conversion, for example an electrolysis, is effected at a current density of 200 mA/cm² or more, advantageously 250 mA/cm² or more, more advantageously 300 mA/cm² or more, even more advantageously 350 mA/cm² or more, especially at more than 400 mA/cm². The electrochemical conversion is advantageously effected at a pH of pH=6-14, advantageously at a pH between 10 and 14.

In the reduction at the cathode, it is especially also possible to obtain ethylene. Thus, this process of the invention is also a process for preparing ethylene.

Additionally disclosed is also use of Cu₄O₃ for the reduction of CO₂, and also the use of Cu₄O₃ in the electrolysis of CO₂.

The above embodiments, configurations and developments can, if viable, be combined with one another as desired. Further possible configurations, developments and implementations of the invention also include combinations that have not been mentioned explicitly of features of the invention that have been described above or are described hereinafter with regard to the working examples. More particularly, the person skilled in the art will also add individual aspects to the respective basic form of the present invention as improvements or supplementations.

The invention is elucidated further in detail hereinafter with reference to various examples thereof. However, the invention is not limited to these examples.

EXAMPLES Example 1

The synthesis of the Cu₄O₃ phase was inspired by a synthesis route (mg range) described in the publication by Zhao et al. (Zhao et al., Chem. Mater. 2012, 24, pages 1136-1142).

A typical synthesis comprises a dissolution of 50 mM Cu(NO₃)₂.3H₂O in 1.1 L of mixed ethanol-DMF solvent (the volume ratio of ethanol to DMF is 1:2). The solution was stirred for 10 min and then transferred to a 1.5 L glass insert that was then inserted into a stainless steel autoclave (BR-1500 high-pressure reactor, Berghof). The autoclave was closed and the reaction mixture was held therein at 130° C. for 24 h. After 24 h, the glass insert with the reaction mixture was removed from the autoclave and cooled down to room temperature by means of an ice bath. The reaction product precipitated out in the glass insert. After cooling, the supernatant was removed from the glass insert and the remaining solid-state product was collected by centrifuging and washing three times with ethanol. The powder obtained was first dried under an argon stream and then dried under reduced pressure. Finally, the powder was stored in a glovebox under inert atmosphere.

An x-ray diffractometry (XRD) analysis of the powder prepared showed the presence of the following phases, as shown in FIGS. 20 and 21: Cu₄O₃ (reference numeral 13), Cu₂O (reference numeral 11) and Cu (reference numeral 12). FIG. 20 is a plot of the angle 20 (coupled 2 theta/theta, WL=1.54060) against the number of pulses I. FIG. 21 is a plot of the angle 20 against the root of the intensity (I^(1/2)) in root counts (C^(1/2)). A quantitative phase analysis was conducted. About 40% by weight of the powder obtained was Cu₄O₃; the remainder was Cu₂O with traces of copper. SEM images of the powder obtained are shown in FIGS. 22 and 23.

A gas diffusion electrode (GDE) containing Cu₄O₃ as catalyst for CO₂ electroreduction was prepared as follows. The previously synthesized powder that contained Cu₄O₃ was cast onto a gas diffusion layer (GPL; Freudenberg H23C2 GDL) from solution, as follows. The binder used was an ionomer (e.g. AS4 from Tokuyama). The ionomer solution is added to the powder containing Cu₄O₃ catalyst particles that has been dispersed in 1-propanol beforehand. The amount of the catalyst powder used depends on the desired catalyst loading, but is generally set for a mass coverage on the gas diffusion layer of between 1 mg/cm² and 10 mg/cm², e.g. here by way of example 4.5 mg/cm². The dispersion was then left in an ultrasound bath for 30 min, whereupon a homogeneous catalyst ink was formed. After the ultrasound treatment, the catalyst ink was poured on and dried in an inert atmosphere (argon).

The electrochemical performance of the GDE containing Cu₄O₃ as catalyst was tested in the electrolysis setup described hereinafter. For this purpose, a stacked three-chamber flow cell was used. The first chamber, which was used as gas supply chamber, was separated from the second chamber by the GDE. The second and third chambers respectively contained a catholyte and an anolyte and were separated by a Nafion 117 membrane. The electrolytes were pumped through the cell in two separate cycles. The anode space was filled with 2.5 M KOH and had an IrO₂-containing anode. For the cathode space, the GDE was used as cathode and 0.5 M K₂SO₄ as electrolyte. The counterelectrode used was a solid, IrO₂-coated Ti plate. The cell was equipped with an Ag/AgCl/3M KCl reference electrode. For potentiostatic measurements, the cathode was connected as working electrode.

In order to demonstrate the superior activity and selectivity for ethylene, the GDE comprising Cu₄O₃ that was produced above was compared with two other GDEs that contained copper particles (Roth) and Cu₂O particles as catalysts. All three GDLs were prepared by the same method as described above. Copper and Cu₂O were selected because they currently represent the state of the art for the reduction of CO₂ to higher hydrocarbons, including ethylene. The experiments were conducted in potentiostatic electrolysis mode, meaning that the cell potential was kept constant during the experiment. Gaseous products were analyzed with a Thermo Scientific Trace 1310 gas chromatograph.

The results of the electrochemical measurements are shown in FIGS. 24 and 25.

As apparent from FIGS. 24 and 25, the Cu₄O₃-containing GDE showed a maximum selectivity of 27% Faraday efficiency (FE) for ethylene at 1.05 V (versus Ag/AgCl) and a current density J of 420 mA/cm². At the same potential, the Cu— and Cu₂O-containing GDEs showed less than 2.5% FE for ethylene at an overall current density of 20 mA/cm2. This shows that a GDE containing Cu₄O₃ shows a 10-fold increase in the ethylene FE at distinctly higher possible overall current densities (more than 20-fold) compared to GDEs comprising Cu and Cu₂O. In order to further illustrate the distinct improvement in activity of the Cu₄O₃ phase by comparison with Cu and Cu₂O, the specific current density for ethylene formation for these three copper phases was plotted against the cathode potentials tested, as shown in FIG. 26. At a cathode potential of −1.05 V (versus Ag/AgCl), GDE with Cu₄O₃ shows a 1000-fold increase compared to the Cu₂O GDE and a 100-fold increase compared to the Cu GDE.

Further gaseous products detected are: CO, CH₄, C₂H₆ and H₂. The FE values for these products are shown in FIGS. 27 to 29, in which the FE values are plotted against the cathode potential for all gaseous products detected. It is of interest that the GDE with Cu₄O₃ was the only one capable of reducing CO₂ to C₂H₆ (albeit only in small amounts, less than 0.5% FE).

Example 2

A GDE containing Cu₄O₃ as catalyst was likewise tested in a double-membrane test setup according to FIG. 17. The GDE was produced as described above. 0.5 M H₂SO₄ was used as electrolyte between the anion exchange membrane AEM (Sustainion x37-50 membrane) and the cathode exchange membrane CEM (Nafion 117 membrane), and as the electrolyte that circulated in the chamber behind the anode. The measurements were conducted in galvanostatic mode, meaning that the GDE was tested at various constant current values. The counterelectrode used was a solid IrO2-coated Ti plate. The cell was equipped with an Ag/AgCl/3M KCl reference electrode. For the galvanostatic measurements, the cathode was connected as working electrode. Since H₂SO₄ was used as electrolyte overall, the pH during the experiment was close to zero. This experiment shows the stability of the Cu₄O₃ phase under extreme acidic conditions (relatively high current densities) in a novel double-membrane setup with a zero-gap anode (CEM directly on the anode surface) and a zero-gap cathode (AEM directly on the cathode surface). The results of the measurements are shown in FIGS. 30 and 31, with FIG. 30 showing the FE values for all products depending on the current density, and FIG. 31 the FE for ethylene. In addition, it was observed that alcohols such as ethanol and propanol were also obtainable.

The present disclosure is of the use of the comparatively rare Cu₄O₃ phase as catalyst for CO₂ reduction, where it is possible to increase the activity and selectivity of a gas diffusion electrode (GDE) containing Cu₄O₃ by comparison with a GDE containing a Cu₂O phase. To date, the Cu₄O₃ copper oxide phase has never been studied as a possible catalyst for CO₂ reduction. The Cu₄O₃ phase shows relatively high stability and can, as shown by x-ray diffractometry, be clearly distinguished from Cu₂O and CuO on account of the different crystal structure. In a formal sense, the oxidation state of copper in this structure is 1.5. The synthesis of Cu₄O₃ was scaled up in the present context, by comparison with literature methods, from a few milligrams to a 10 g scale. A gas diffusion electrode containing Cu₄O₃ as electrocatalyst for CO₂ reduction was studied for the first time and showed high activity and selectivity for ethylene. 

1. An electrode, comprising: Cu₄O₃.
 2. The electrode as claimed in claim 1, wherein the Cu₄O₃ is present in an amount of 0.1-100% by weight based on the electrode.
 3. The electrode as claimed in claim 1, wherein the Cu₄O₃ has been applied to a support.
 4. The electrode as claimed in claim 3, wherein the Cu₄O₃ has been applied with a mass coverage of at least 0.5 mg/cm².
 5. The electrode as claimed in claim 1, wherein the electrode is a gas diffusion electrode.
 6. An electrolysis cell, comprising: an electrode as claimed in claim
 1. 7. A process for producing an electrode comprising Cu₄O₃ on a substrate, the process comprising: preparing a mixture comprising Cu₄O₃ and optionally at least one binder, or providing a powder consisting of Cu₄O₃, applying the mixture comprising Cu₄O₃ or the powder consisting of Cu₄O₃ to a support, preferably in the form of a sheetlike structure, and dry rolling the mixture comprising Cu₄O₃ or the powder consisting of Cu₄O₃ onto the support.
 8. The process as claimed in claim 7, wherein the preparing of the mixture comprises mixing for 60-200 s.
 9. The process as claimed in claim 7, wherein the rolling application is effected at a temperature of 25-100° C.
 10. A process for producing an electrode comprising Cu₄O₃ on a substrate, the process comprising: providing a support, applying a suspension comprising Cu₄O₃ and optionally at least one binder to the support, and drying the suspension; or providing a support, and applying Cu₄O₃ or a mixture comprising Cu₄O₃ from the gas phase.
 11. The process as claimed in claim 10, wherein the support is a gas diffusion electrode or gas diffusion layer.
 12. The process as claimed in claim 7, wherein the at least one binder is present in the mixture.
 13. The process as claimed in claim 12, wherein the at least one binder is present in the mixture in an amount of >0 to 30% by weight, based on the total weight of Cu₄O₃ and the at least one binder.
 14. A process for producing the electrode of claim 1, the process comprising: producing a powder comprising Cu₄O₃; and rolling the powder out to give an electrode.
 15. A process for electrochemical conversion of carbon dioxide and/or carbon monoxide, the process comprising: introducing carbon dioxide and/or carbon monoxide at the cathode into an electrolysis cell comprising an electrode as claimed in claim 1 as cathode and reduced.
 16. A process for reduction of CO₂ and/or CO, comprising: using the electrode of claim
 1. 17. A process for the electrolysis of CO₂ and/or CO, comprising: using the electrode of claim
 1. 18. The electrode as claimed in claim 2, wherein the Cu₄O₃ is present in an amount of 40-100% by weight based on the electrode.
 19. The electrode as claimed in claim 2, wherein the Cu₄O₃ is present in an amount of 70-100% by weight based on the electrode.
 20. The process as claimed in claim 9, wherein the rolling application is effected at a temperature of 60-80° C. 